Immutable DB validation: warn user when about to truncate a large part of the DB

When the validation finds an invalid block early in the immutable DB (either due to data corruption or due to clock changes, input-output-hk/ouroboros-consensus#722), it might be nice to ask the user before deleting a large part of the database.

Note that this will need to be done in such a way that Daedalus can present this choice to the user.

Setting to medium priority because although the _real_ causes for such a truncation (wrong clock, data corruption) would be very rare, if we decide to truncate due to a _bug_ in our code, it would be good if the user gets a warning ahead of time.

Start by locating the immutable DB validation path that detects an invalid block and the code that decides to truncate the database. Trace how a user choice could be exposed to Daedalus; done means a large truncation is preceded by a warning and Daedalus can present and handle that choice.

Written by the indexing model from the issue text.